This article appeared in the January 1990 issue of Code One Magazine.

Any story about what goes on at Brooks AFB in San Antonio, Texas, has to begin with the human centrifuge. It just has to. The centrifuge is big, flashy, and fast. It's high-tech reminiscent of a carnival ride. And like the biggest roller coaster in a theme park, it's the main attraction. On entering the blue and white roundhouse that surrounds the spotlessly clean machinery, you almost expect an operator to ask for a ride ticket and an underpaid high-school student to strap you into the gondola.

But this is no carnival ride. You need a special physical before they let you on. And you may not want to get on. The forces generated by this large, white spinning arm can temporarily disfigure—literally press your eyeballs back into your head. As the arm accelerates, your face will tighten as the skin slides back towards your ears. Your head will either sink deep into your chest or become uncontrollably lodged against the headrest. The human neck was not designed to support a 200-pound head. At some point, your vision will tunnel. Then turn gray. Then black out. Then you lose consciousness and let go of a sweaty stick, which stops the ride. Sirens go off. The hatch opens. You regain consciousness, leap out, and kiss the floor.

"Congratulations, you just survived 4.2 g's," comes matter-of-factly over the intercom. That figure is typical for what is termed a "relaxed" g tolerance, that is, the g tolerance that comes without executing an anti-g straining maneuver. The figure is, however, on the wimpy side, considering the centrifuge can do 50 g's.

The human body wasn't designed to survive 50 g's. In fact, it doesn't much enjoy three g's. Physical tolerance to g forces is largely determined by the circulatory system. The heart has to work extra hard to supply enough oxygen-enriched blood to the brain to counter a force of three or four g's. It's mostly a matter of physics" When the heart can no longer overcome the downward force on the column of blood that leads to the brain, consciousness slides away. The eyes are most sensitive to losses of blood pressure. The tiny blood vessels that wrap around from the retinal artery behind the eye react first. This accounts for the initial tunnel vision under g forces. The tunnel vision and the subsequent blackout turn out to be natural visual warnings for the oncoming loss of consciousness. About one g before losing consciousness, your eyes tell your brain that you'd be better off in a Venetian gondola.

This handy anatomical warning system is lost when g forces increase at a fast rate (what is called a high g-onset rate). For example, a subject will go straight from consciousness to unconsciousness — no tunnel vision or blackout — when acceleration increases from 2 to 6 g's in one second. Before the late '70s, high onset rates were not, in general, an operational concern for fighter pilots. This changed with the introduction of the F-16. Soon after, the centrifuge at Brooks was upgraded to be more in line with the performance characteristics of the new airplane. Four 250-hp motors, the largest motors that would fit into the facility, were installed. The new powerplants can swing the gondola from 1 to 13 g's in two seconds flat.

If you were to make a list of pleasurable sensory experiences, going from 1 to 13 g's in two seconds would not be on it. Some pilots speculate, not without reason, that fighter aircraft will become less and less hospitable. What's to be done about this dilemma? Instead of trying to better accommodate pilots, perhaps we should eliminate them. Just get them out of the cockpit and hand over the controls to someone on the ground or to some g-insensitive robot. Such a simple solution draws a quick reply from Dr. Carter Alexander, chief of the Crew Technology Division at the USAF School of Aerospace Medicine at Brooks. "Humans may be the limiting factor, but we must remember that they are the enabling factor as well," insists Alexander, who is in charge of the centrifuge lab. "The two provide an unexcelled combination."

This human/machine combination is often a balancing act between comfort and efficiency. The balance is nothing new, it's been around about as long as machines have. For most of us, though, the balance becomes noticeable only when it is not addressed well. Take an elevator, which could be designed to get passengers from the first to the fort-second floor very fast, in a few seconds. The result would be highly efficient, but awfully uncomfortable. Imagine lying on the floor of an elevator grunting in an anti-g straining maneuver. Most of us (test pilots excluded) would gladly trade a little inefficiency for comfort and social acceptability.

Such luxuries don't exist in jet fighters-where a smooth, painless flight translates into "easy target." The balance must be pushed to the limit — the human limit — for survival. The job of the centrifuge is to explore and quantify these limits of human physical tolerance. But that's only part of the story.

The relationship between man and machine is not necessarily one of friend against foe. Technology plays a double role: it can protect a pilot from the hazards it produces. Besides exploring limits, the centrifuge can be used to test new devices and techniques that can extend these limits, allowing pilots to sustain higher g forces or g forces for longer periods of time. "The idea," says Dr. Alexander, "is to match the physiological envelope of man to the operational envelope of the aircraft, so the combination can reach its full potential. "

One program at Brooks, Combat Edge, has combined several devices and techniques into a single system. In simple terms, the system does the breathing and straining maneuver for the pilot. This is achieved, in part, by an automatic mask-tensioning system built into the helmet. With a tight mask, air can be forced into the lungs (what is called positive-pressure breathing). An inflatable vest does the exhaling by providing the necessary counter pressure to the chest surface. The system works well. By the end of 1992, F-16 and F-15 pilots will be equipped with it.

The centrifuge has been used to test lightweight helmets, advanced g suits, and a valve that controls g suit inflation. This last device is sensitive to the magnitude of the g force and the g-onset rate. A "smart mask" that can detect when a pilot is incapacitated has been tested as well. The mask provides physiological status (e.g., pulse rate and how efficiently oxygen is being delivered) of the pilot to the flight control computer and flight recorder. It can also warn the pilot of fatigue or hyperventilation.

How does Alexander and his staff plan to deal with upcoming, and even less hospitable, aircraft designs? "Our bag of tricks is still full," he says. "For example, there's seat position." In Alexander's description of a cockpit of the future, the pilot's seat will articulate according to g load. The seat will slant the pilot at an optimum angle for experiencing a tight turn. "One of our missions is to look twenty years ahead, to anticipate technologies and their effects on the human system," says Alexander.

The centrifuge also has more immediate applications. The facility at Brooks has been used to train new fighter pilots. In a one-day course, new fighter pilots who are heading to their first squadron assignment are lectured on the physiology of high-g flight. This lecture is supplemented with several rounds on the centrifuge, where the pilots are put through various g-force scenarios. Subjects can refine their straining techniques by reviewing videotapes of each run. The training reinforces the pilots' understanding of what can happen if they don't properly perform a straining maneuver.

Such training is effective and inexpensive, especially considering the cost of losing consciousness in an actual flight. Many lives (and multimillion-dollar planes) have been saved with this training, which is now being conducted at the Air Force's newest centrifuge at Holloman AFB in New Mexico.

Despite appearances, the centrifuge is no carnival ride. The huge white arm in the blue-and-white roundhouse plays a pivotal role in a delicate balance between man and machine.